1. Field of the Invention
The present invention relates to an apparatus and method for estimating a delay spread of a multi-path fading channel, which is applicable to an Orthogonal Frequency Division Multiplexing (OFDM) system, and more particularly, to an apparatus and method for estimating a multi-path maximum delay time in an OFDM system, which is designed to have an output characteristic proportional to a delay length by using a channel estimation value difference between adjacent channels at locations of pilot signals. The apparatus and method determines a maximum power response location by detecting only a maximum power value without setting a threshold value, thereby estimating a maximum delay path correctly.
2. Description of the Related Art
In general, Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme suitable for high-speed data transmission in wired/wireless channels. Recently, a variety of high-speed communication systems have adopted OFDM as a transmission scheme. If a single carrier scheme is used for high-speed data transmission with a short symbol period in a wireless communication channel having multi-path fading, inter-symbol interference (ISI) becomes severer and the complexity of a receiving end greatly increases. On the other hand, a multi-carrier scheme can extend a symbol period on each sub-carrier by the number of sub-carriers, without decreasing a data rate. Therefore, the multi-carrier scheme can cope with severe frequency selective fading channels, which are caused by multi-path, by using a simple 1-tab equalizer.
Furthermore, the OFDM scheme has high frequency efficiency because it uses multiple orthogonal carriers. The modulation and demodulation of the multiple carriers in a receiving end and a transmitting end are the same as the execution of Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT), respectively. Therefore, high-speed modulation and demodulation can be implemented using Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT), respectively. Such an OFDM scheme was adopted as standards for IEEE 802.11a, IEEE 802.16a/d, DAB/DMB, and DVB-T because it is suitable for high-speed data transmission.
OFDM signals experiencing multi-path fading channels are affected by frequency selective channels in the frequency domain. Therefore, in order for stable channel estimation, pilot signals are transmitted at specific sub-carrier locations so that the OFDM signals can be adapted to variation of frequency-domain channels. In this case, intervals of the pilot signals are designed considering a delay spread of a multi-path fading channel. If the delay spread increases and the channel extremely varies within the predefined intervals of pilots, channel estimation error occurs and demodulation performance is greatly degraded.
In order to minimize the performance degradation, the characteristics of the multi-path fading channel should be known in the channel estimation process. Among them, information on the channel delay spread is most important.
FIG. 1 illustrates locations of pilot signals in an OFDM system.
In the OFDM system of FIG. 1, a transmit signal with multiple sub-carriers includes pilot signals carried on a specific carrier in order for estimating channel distortion caused by a multi-path fading and acquiring a frequency and time synchronization of a receive signal.
As illustrated in FIG. 1, the pilot signals are carried on sub-carriers spaced at constant intervals M within the transmit signal, and sub-carriers other than the pilot sub-carriers are used for data transmission.
In an OFDM receiver using the pilot signals carried on the specific sub-carriers at constant intervals, an initial channel estimation is directly performed only at the locations of the pilot signals, and a channel estimation for data signals between the pilot signals is performed using channel estimation values of the pilot signals by an interpolation process or the like.
Meanwhile, the initial channel estimation values at the locations of the pilot signals are also used for estimating a delay spread of a multi-path fading channel, and the accuracy of the interpolation process can be increased by the estimated delay spread value.
FIG. 2 is a block diagram illustrating an apparatus for estimating a delay spread in an OFDM system according to the related art. Referring to FIG. 2, the OFDM system according to the related art includes an OFDM transmitter 10, a wireless channel, and an OFDM receiver 20. The OFDM receiver 20 includes an RF receiving unit 21 converting an RF transmit signal into an IF signal, an FFT unit 22 performing an FFT process to transform the time-domain IF signal into a frequency-domain IF signal, a channel equalization unit 23 estimating and compensating channel distortion of a signal output from the FFT unit 22, and a data demodulation unit 24 demodulating an output signal of the channel equalization unit 23 into the original data.
In the operation of the channel equalization unit 23, an initial channel estimator 23A estimates channel distortion of pilot sub-carriers from the output signal of the FFT unit 22. A channel interpolator 23B performs an interpolation process to calculate channel estimation values of data between the pilots by using the channel estimation values that are obtained at the locations of the pilot signals by the initial channel estimator 23A. A channel equalizer 23C compensates channel distortion by using the channel estimation values output from the channel interpolator 23B. A delay spread estimator 23D converts the channel estimation values, which are obtained at the locations of the pilot signals by the initial channel estimator 23A, into time-domain values, and estimates a maximum delay location by estimating impulse responses.
The time-domain receive signal input to the FFT unit 22 can be expressed as the following Equation (1):
                              y          n                =                                                            h                n                            ×                              x                n                                      +                          w              n                                =                                                    ∑                                  l                  =                  0                                                  L                  -                  1                                            ⁢                                                h                  l                                ·                                  x                                      n                    -                    l                                                                        +                          w              n                                                          (        1        )            where yn is the receive signal, hn is the impulse response of the channel, xn is the transmit signal, and wn is noise component.
In this case, the time-domain receive signal is transformed into the frequency-domain signal through the FFT process of the FFT unit 22. The frequency-domain signal is expressed as the following Equation (2):Yk=HkXk+Wk  (2)where Yk, Hk, Xk and Wk represents the Fourier-transformed values of yn, hn, xn and wn, respectively.
Next, the initial channel estimator 23A estimates the initial channel values at the locations of the pilot signals. The estimation process is expressed as the following Equation (3):Hk=Yk/Xk=Hk+Wk/Xk  (3)
Next, the delay spread estimator 23D includes an IFFT unit 23D-1, an effective impulse response estimation unit 23D-2, and a maximum delay location estimation unit 23D-3.
The IFFT unit 23D-1 performs an IFFT process on the initial channel estimation values, and outputs the IFFT-processed initial channel estimation values to the effective impulse response estimation unit 23D-2. The effective impulse response estimation unit 23D-2 detects the impulse responses having power higher than a threshold value, which is set to noise power, and outputs the detected impulse responses to the maximum delay location estimation unit 23D-3. The maximum delay location estimation unit 23D-3 detects an impulse response value having the greatest sample index value among the detected effective impulse response values.
The above-described process is expressed as the following Equation (4):τmax=max[n]l|hn2|>Γ  (4)
In the above process, the detected impulse response location is a value that is determined as a maximum delay path by the delay spread of the multi-path fading channel.
The delay spread estimation of the delay spread estimation unit according to the related art uses the impulse response of the basic channel, but a process of setting the impulse response value through the estimation of noise power is followed. Therefore, when the power of the impulse response value for the maximum delay path is similar to the noise power, it is difficult to detect the effective impulse response.